Large circular constriction contractions scattered along colon.
Caused by the combined contraction of circular muscles (about 2.5 cms) & longitudinal muscle fibers( which are arranged into 3 longitudinal strips called (Tenae coli)cause the colonic wall to bulge outside into baglike sacs causing the appearance of haustrations .
Help in mixing colonic contents & absorbing water & electrolytes from the wall so concentrating it to semifluid, mush, semimush, solid & finally hard food residues called stool.
2.propulsive or mass Ms:
forward Ms. Help to push stool towards the rectum & initiating the defecation reflex.
more abundant in the transverse & sigmoid colon, stimulated by distention or irritation of colon.
the sensation of its presence is transmitted to the myenteric plexus & through parasympathetic pelvic nerves to the spinal cord to initiate colonic contractions .
External sphincter relaxation occur through skeletal motor nerves starting defecation when the situation is proper as there is higher centers control from conscious cortex over the whole defecation reflex.
Reflexes in the colon and rectum Mass movements + + Food in stomach Food in duodenum Faeces Defaecation reflex Colon rectum anus Ach Spinal cord IAS EAS FAECES + Distention - VIP ATP + - Pudendal nerve
The Defecation Reflex: Summary Removes undigested faeces from the body. Stretch receptors in GIT wall detect distension of rectum. Parasympathetic reflex causes contractions of the sigmoid colon & rectum + relaxation of internal anal sphincter. External anal sphincter (under voluntary control) consciously relaxed if appropriate.
Secretary functions of GIT: Of 2 types: 1.Enzymes helping digestion. 2.Mucous for lubrication & protection of GIT mucosal surfaces from excoriation. Anatomical types of secretary glands: 1.Mucous glands: 2 types: a.single cell (goblet cells). b.complex cell mucous glands. 2.Crypts of Liberkhan: deeper & contains specialized secretary cells. 3.Deep tubular glands in stomach & upper duodenum secreting acid & pepsinogen. 4.Complex glands like salivary ,pancreatic & hepatic glands.
1.Local contact of food with GIT mucosal surfaces activating enteric nervous system through:
B. tactile irritation.
specially through the parasympathetics (vagi & other cranial parasymp. Nerves) stimulating eso.,stomach,pancreas, Brunner glands in duodenum & glands of distal colon,while secretion in the remainder of SI & 1 st 2/3 of large intestine(LI) is stimulated by local myenteric nerves & hormones locally in each segment.
The sympathetics also slightly increase the glandular secretion but through vascular constriction reduces secretion as an overall effect.
Daily secretion is 1 liter in a rate of 0.5 ml/min. during day & very little during sleep.
It contain 2 major types of secretions:
1.serous secretion containing ptyalin ,an alpha-amylase for digesting starch.
2.mucous secretion for lubrication.
salivary glands consist of acini & ducts ,in the acini there is primary secretion of ptyalin,mucous & extracellular fluid while in the ducts there is K+ & HCO3- secretion & active Na+ reabsorbtion & passive Cl- reabsorbtion,so as a result there is high K+ & HCO3- & low Na+ & Cl- in the saliva.
Stemulation through parasympathetic NS(PNS) from salivary nuclei located at the brain ponto-medulary junction excited by taste(specially sour) & tactile stimuli(as presence of smooth objects in mouth),
inhibited or stimulated by higher centers specially the appetite center located close to PNS center in the anterior hypothalamus which function in response to signals from taste & smell areas of the cerebral cortex or amygdala.
salivation also occurs in response to reflexes in the stomach& upper intestine by the presence of irritating food or nausea since saliva has diluting & acid neutralizing effect.
no enzymes , only mucous secreted by simple & complex mucous glands.
CONTROL OF SALIVARY SECRETION cerebral cortex salivary centre in medulla autonomic nerves salivary glands salivary secretion pressure receptors and chemoreceptors in the mouth other inputs:smell& taste centers conditioned reflex simple reflex
Done by Hcl as it is inactive in an alkaline medium.
Regulation of gastric acid secretion:
stimulated by 3 hormones:ACH(PNS),gastrin & Histamine:
1.PNS (vagus) stimulation :secreting ACH which can be blocked by anticholinergic drugs pirenzepen used for peptic ulcer(PU) therapy.
2.Gastrin release from antral glands which can be blocked by proglumide.
3.Histamine release stimulating H2 receptors which can be blocked by H2 blockers as cimetidin(Tagamet).
The 3 above hormones secret Hcl through activation of the proton pump(H+-K+ ATPase)which is the final common pathway in acid secretion which can be blocked by omperazole ,an effective therapy for peptic ulceration & hyperacidity.
The Secretions of Hydrochloric Acid Figure 24.14
1.Acid feed back(FB) inhibition in the presence of excess acid when the S (PH) becomes 3 or less.
In patients with (PU) this (FB) inhibition is abnormal so Hcl continue to be secreted in spite of very high acid& low PH in the stomach leading to PU.
2.Through the enterogastric reflex in the presence of excess acid,fat & protein breakdown products,hyperosmolar fluid ,distention or any irritating factor in the upper SI which cause the release of several inhibitory intestinal hormones as secretin,CCK,Gastrin inhibitory peptide & somatostatin.
3.Interdigestive period: in this period between meals the S secrets few mls. Of gastric juice containing little enzymes, more mucous & moderate amounts of HCO3 called non-oxyntic type of secretion .This interdigestive type of secretion may change with high enzyme-acid content in patients with PU & those with emotional upsets.